1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image corresponding to information to be recorded by scanning a photosensitive member with a light beam.
2. Description of the Prior Art
For the purpose of obtaining a visible image from computer output signals or electrical signals from image information output means, there is already known an image forming apparatus in which an electrophotographic photosensitive member is scanned with a laser beam modulated corresponding to the information to be recorded. In such apparatus, the laser beam scans said photosensitive member in a direction perpendicular or substantially perpendicular to the moving direction of said photosensitive member. The moving direction of said beam and that of said photosensitive member are respectively called the principal scanning direction and the auxiliary scanning direction. Since the photosensitive member is constantly displaced, the starting point of each scanning stroke with the light beam in the principal scanning direction moves in the auxiliary scanning direction. In such scanning system the quality of the obtained image becomes deteriorated if the beam position relative to the photosensitive member is dislocated from the determined position, namely if the pitch between a scanning line, constituted by the beam trajectory on the photosensitive member, and a succeeding scanning line becomes different from the predetermined pitch.
Such inconvenience has only been avoided by employing a strongly-built structure and by smoothly rotating the photosensitive member with a strong torque. This is due to the fact that, in the case of a resolution of the order of 10 dots/mm, the fluctuation in the scanning pitch can be maintained at less than 100 microns by the use of a firmly built structure in the optical system and in the drive mechanism for the photosensitive member and the deterioration in the image quality is not visually apparent because of the limited number of dots per millimeter.
Such fluctuation in the scanning pitch will however present a problem if a higher speed or a higher resolution is required in such apparatus, since vibration inevitably resulting from the mechanical drive system composed of the drive source, transmission system etc. gives rise to a distortion in the optical system or a slight vibration of the photosensitive member in the displacing direction thereof due to the fluctuation in the rotating torque, thus generating fluctuation of the beam position relative to the photosensitive member. The effect of such fluctuation on the image is not serious if the amount of said fluctuation does not exceed 10% of the designed scanning pitch, but the effect cannot be ignored if said amount becomes comparable to said pitch. Naturally this problem becomes more serious as the apparatus functions at a higher speed, or as the image is formed with a higher density, i.e. with a smaller scanning pitch.
As a numerical example, let us consider a case with a beam diameter of 120 microns and a designed pitch of 83 microns between the scanning lines, corresponding to 12 dots/mm. As the spatial intensity of a beam spot on the photosensitive member follows the Gaussian distribution in the scanning of a constant pitch with a single-mode laser beam, the integrated beam intensity in the auxiliary scanning direction assumes the form shown in FIG. 1, in which the light intensity is represented in a linear scale normalized to the maximum intensity. Also FIG. 2 shows the intensity distribution in case the scanning lines are alternately turned on and off. In order to obtain the highest resolving power on the image, the image-forming process conditions such as the sensitivity, charge, developing bias etc. should be so regulated that the white and black areas are reproduced in the image with the beam exposure shown in FIG. 2. Also in order to obtain a satisfactory image not showing the traces of scanning lines, the above-mentioned process conditions should be so regulated that the beam exposure shown in FIG. 1 provides a solid white or black image respectively in case of a positive-working system in which the exposed area forms a white area or in case of a negative-working system in which the exposed area forms a black area. The ripple in the light intensity becomes larger with the increase in the pitch L of the beam scanning lines. Said distance L can be selected to be larger in the negative working system since the black scanning traces on a white background are visually more unpleasant than white scanning traces on a black background. The above-mentioned objective can be attained for example in the negative-working system by selecting the aforementioned process conditions in such a manner than an area of the photosensitive member exposed with a light intensity exceeding 0.5 is reproduced as black while an area exposed with a light intensity lower than 0.5 or not exposed to the light is reproduced as white. Even when there is a continuous change in density between white and black, a sufficiently high signal-to-noise ratio can be obtained by selecting the intermediate density in the vicinity of 0.5 in the light intensity.
Now, in case the displacement of the beam position in the auxiliary scanning direction on the photosensitive member is influenced by a sinusoidal vibration with a maximum amplitude of 120 microns and a cyclic period equal to 10 scanning cycles, the integrated light intensity shown in FIG. 1 changes to a form represented in FIG. 3. Further it changes to a form shown in FIG. 4 in case the amplitude of the vibration is increased to 600 microns. Similarly the integrated light intensity shown in FIG. 2 changes as shown in FIG. 5 and 6 under the influence of the above-mentioned sinusoidal vibration. Also mutually overlapping plural vibrations provide so-called beat phenomenon.
For example the photosensitive member exposed with an intensity distribution shown in FIG. 3 acquires the surface potential V of a form as shown in the third quadrant of FIG. 7, in which the curve (a) represents a 4-times higher sensitivity than for the curve (b). Stated differently these curves indicate that the situation cannot be improved even with a beam of a 4-times increased intensity.
On the other hand it is extremely difficult to maintain the aberration in the auxiliary scanning direction of the beam position on the photosensitive member within 10.mu., since the optical system, including the scanning means such as a rotary polygonal mirror, the beam focusing means such as an f.theta. lens and eventually a deflecting mirror, must be firmly supported and protected from the vibration during the beam scanning operation. In a scanning system having a deflecting mirror positioned between an f.theta. lens and a photosensitive member and at a distance of 200 mm from said photosensitive member, the angular aberration of said mirror must be maintained within 1.7 minutes in order to reduce the error of the beam position in the auxiliary scanning direction to 100.mu. or less on the photosensitive member. Besides it is not easy to maintain the fluctuation in the rotation of the photosensitive member at 100.mu. or less in case the photosensitive member is in contact with a heavy load such as a blade cleaner. These causes deteriorate the resolving power of the reproduced image and give rise to irregular beam scanning patterns in said image.